1. Field of the Invention
This invention relates generally to methods of manufacturing a stent and, more particularly, to methods of manufacturing a stent from a polymer tube.
2. Description of the State of the Art
Stents function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. A “lumen” refers to a cavity of a tubular organ such as a blood vessel.
A stent has a cylindrical shape and includes a pattern with a number of interconnecting structural elements or struts. Some stents are designed so that they may be radially compressed (crimped) and radially expanded (to allow deployment). A stent can be fabricated from a tube that has been laser cut to form a stent pattern.
The stent must be able to satisfy a number of mechanical requirements. First, the stent must withstand structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a lumen. Therefore, a stent must possess adequate radial strength. Radial strength, which is the ability of a stent to resist radial compressive forces, is due to strength and rigidity around a circumferential direction of the stent. Radial strength and rigidity, therefore, may also be described as, hoop or circumferential strength and rigidity. Once expanded, the stent must adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including cyclic loading, which is induced by a beating heart.
The stent can be manufactured from a polymer tube. To increase the strength and rigidity of the polymer tube, the polymer tube can be expanded radially and/or axially so as to orient the polymer molecules of the tube in a manner that provides greater strength and rigidity along the direction of expansion. The polymer tube can be expanded in a tubular mold in order limit the amount of expansion. Typically, the polymer tube is heated within the tubular mold to allow for the desired expansion. Selected segments of the polymer tube can be heated, which transfers heat to segments of the polymer tube that are to be expanded.
Highly uniform radial expansion of the polymer tube is often desired so that a stent that is eventually formed from the polymer tube will have highly uniform mechanical properties, such as strength and rigidity.